Mass of the solution = mass of solute + mass of solvent
Given that the mass percent of sodium sulphate solution is 20%.
Let the mass of sodium sulphate which is required to prepare this solution be x grams.
Given that the mass of water is 100 g.
20 = [x/(x + 100)] * 100
20/100 = [x/x + 100]
1/5 = x/x + 100
x + 100/5 = x
x + 100 = 5x
5x - x = 100
4x = 100
x = 100/4
x = 25
Thus, 25g of sodium sulphate is required.